More than 26,000 transport users in London have made use of badges for people with hidden health problems.
Transport for London (TfL) launched the blue "Please offer me a seat" badge in April for people who find it difficult to stand.
The scheme has since been introduced on Greater Anglia trains as well as the New York subway.
Cancer survivor James McNaught said the badge helped him on his daily commute.
Mr McNaught, who took part in the trial leading to the scheme's introduction, said the badge was especially useful to him "in the evenings, as my energy levels can fall very low later on in the day".
Knowing he had it meant he could focus on his job during the day rather than "than worry about my journey home".
Mr McNaught previously made his own "cancer on board" badge after chemotherapy on his throat left him unable to speak and doses of morphine made him appear drunk.
TfL said the scheme was created in response to comments from its customers who struggled to get a seat because their need was not obvious.
There is no set definition of conditions that qualify for the badge and card, but TfL says the system will be based on trust - as with the existing "Baby on board" badge scheme.
A survey of about 400 badge users carried out in October showed that 78% found it easier to get a seat and 95% would recommend the scheme to a friend, TfL said.
Mark Evers, chief customer officer for London Underground, said the scheme "made a real difference to passengers who need a seat but may not have felt confident enough to ask for one".
Source (BBC): http://www.bbc.co.uk/news/uk-england-london-42427867